1. Field of the Invention
The present invention relates to a logarithmic amplifier. The logarithmic amplifier is a circuit outputting input signal compressing to logarithm, and is used for various analog electronic circuits. The invention relates particularly to a logarithmic amplifier removing dependency on temperature.
2. Description of the Related Art
A conventional logarithmic amplifier is known, which uses matter that forward direction voltage Vf and forward direction current If of a diode are in logarithmic relation. A circuit diagram of the conventional logarithmic amplifier is shown in FIG. 9. In FIG. 9, input voltage Vi is supplied to an inversion input terminal xe2x80x9cxe2x88x92xe2x80x9d of an operational amplifier 100 through a resistor R, and a non-inversion terminal xe2x80x9c+xe2x80x9d is connected to ground. A diode 101 is taken in negative feedback loop of the operational amplifier 100. Forward direction voltage Vf and forward direction current If of the diode 101 are in the following logarithmic relation.
Vf=Kxc3x97log(Tf/Is)xe2x80x83xe2x80x83(1)
Here, constant K is a constant determined by Boltzmamm""s constant k, absolute temperature T, electric charge of electron Q, and so on. xe2x80x9cIsxe2x80x9d is saturation current when backward direction voltage is applied to the diode 101.
Since the inversion input terminal xe2x80x9cxe2x88x92xe2x80x9d of an operational amplifier 100 is imaginary earth, the following expression holds letting Ii is current flowing through the resistor R.
Ii=Vi/R=Ifxe2x80x83xe2x80x83(2)
Vo=xe2x88x92Vfxe2x80x83xe2x80x83(3)
The following expression (4) is deduced from the above expressions (1) to (3). That is, output voltage Vo converting input voltage Vi to logarithm is obtained.
Vo=xe2x88x92Kxc3x97log(If/Is)=xe2x88x92Kxc3x97log(Vi/Isxc3x97R)xe2x80x83xe2x80x83(4)
According to the logarithmic amplifier having the above-mentioned constitution, linear characteristics is obtained input voltage Vi in logarithmic scale for horizontal axis, and output voltage Vo for vertical axis as shown in FIG. 10.
However, because constant K has large dependency on temperature in expression (4), there is a problem that sensitivity of output voltage Vo drifts largely by temperature as shown in FIG. 10.
Then, an object of the invention is to provide a logarithmic amplifier removing dependency on temperature as far as possible and realizing stable operation.
The logarithmic amplifier of the invention provides: a plurality of linear amplifiers, to each of which input signal is supplied, having different gains; and a comparator detecting a plurality of different levels of said input signal and outputting level detecting signal corresponding to the individual level, wherein any one of the linear amplifiers among said plurality of linear amplifiers is switched so as to operate by supplying said level detecting signal to said plurality of linear amplifiers.
Because the linear amplifier is small in dependency on temperature, by switching so that any one of linear amplifiers among the a plurality of linear amplifiers operates corresponding to level of input signal, logarithmic converting characteristics of the logarithmic amplifier are obtained approximately and the logarithmic amplifier removing dependency on temperature as far as possible can be obtained. By increasing number of linear amplifiers and levels of input signal detected by the comparator, it is possible to improve logarithmic characteristics.